The present invention relates to a transmitter-receiver and more particularly to a phase locked loop type transmitter-receiver provided with a frequency synthesizer of the phase locked loop type.
A conventional transmitter-receiver provided with a frequency synthesizer of the phase locked loop type will be described with reference to FIG. 1 which shows a circuit diagram thereof by way of example. A reference oscillator 1 equipped with a quartz oscillator X.sub.R applies its reference frequency, for instance, 10.24 MHz, to a fixed frequency divider 2 to divide the frequency into 10 KHz. The offset frequency of No. 1 offset oscillator 15, equipped with quartz oscillator Xo, and the output frequency of a voltage-controlled oscillator 8 are applied to a programmable counter 11 through a mixer 5 and are thereby divided into 10 KHz. The above-mentioned two divided frequencies are applied to a phase discriminator 6 where they are detected; and then the detected signals are fed back via a low pass filter 7 to the above-mentioned voltage-controlled oscillator 8. Thus a closed loop is formed. In such a configuration, however, interference frequencies caused by the intereference between the above oscillators tend to exert adverse influence on the frequency band of the intermediate stage through receiving side mixer 14 and IF mixer 3 during reception; meanwhile, during transmission, the harmonic beat, produced by combining the input frequency, i.e., the reference oscillation frequency and the local oscillation frequency, tends to approach in frequency the output frequency even when the input and output frequencies are considerably different from each other, since the frequency band passed through the transmitting side mixer 12 is comparatively wide, and thus there inherently exists the possibility of the increase in the intensity of spurious emission. Moreover, if two adjacent frequencies, for instance, 17 KHz and 10 KHz, are mixed to synthesize a transmission frequency of 27 MHz, the degrees of the higher harmonics of the respective frequencies will be increased resulting in the increase of the possibility of generating the interfering spurious emission.